The way to reproduce this bug:
1. Prepare two instances of hierarchial design that differ only by reference designators and have the same hierarchial symbol.
2. In the "root" design, insert one of those instances as a symbol, but don't click the right button (that would end the command) yet. You are now in a state where the second instance of that symbol is on the screen, ready to be click-inserted.
3. Now, in the tool window try to change the design file name to the second instance. This is when the program crashes.
Mike.
mkishinevsky@yahoo.com